This invention is concerned with conversion systems and more particularly with analog-to-digital conversion systems which provide a digital output from an analog input.
The general operation of analog-to-digital conversion systems is well-known to those reasonably skilled in the art. For example, in the laboratory environment a series of electrical voltages or currents is generated on an experimental basis, typically by sensing transducers connected to the experiment being performed. The sensed outputs which are usually voltages are then transmitted to a sample-and-hold circuit which continuously tracks the experimental voltage being provided. At a predetermined time, the sample-and-hold circuit is enabled and the experimental voltage from the sample-and-hold circuit is fixed and transferred to an analog-to-digital converter circuit. The analog-to-digital converter circuit includes a digital-to-analog converter which provides a series of selected comparison voltages. When the voltage provided by the digital-to-analog converter is favorably compared to the fixed analog voltage, the process is halted. A digital number corresponding to the selected comparison voltage provided by the digital-to-analog converter is then read out from the control logic to an instrument or computer.
Inherent in the above description is that several comparisons utilizing differential amplifiers and feedback circuits are required. Because of this, offset errors are introduced into the conversion system, these errors being a major problem vis-a-vis accuracy in measuring the analog input. The major source of system offset errors is the combination of internal voltage and current offsets of the input amplifier in the sample-and-hold circuit and the internal voltage and current offsets of the comparator amplifier in the analog-to-digital converter circuit. The major cause of the individual offset voltages and currents would be the mismatch between the base emitter voltages and transistors corresponding to the two separate and distinct inputs provided to each of the amplifiers and between the base currents which bias each of the two transistors.
In the prior art these problems were overcome by utilizing very high precision and expensive amplifiers so as to minimize individual offset errors and consequently system offset errors. Moreover, additional components could be added to adjust out the offset errors, but the problem still remains that the offset errors drift with temperature. As is apparent, the above solutions require more complex circuits as well as greater expense.
It is, therefore, a primary object of this invention to provide an analog-to-digital conversion system which overcomes the above-recited limitation.
It is a further object of this invention to provide an improved analog-to-digital conversion system which not only reduces the number of components incorporated in the system but also utilizes inexpensive circuitry thereby lessening the cost required for an analog-to-digital conversion system.
It is yet a further object of this invention to provide an improved analog-to-digital conversion system which overcomes the errors introduced by the individual components in an analog-to-digital conversion system.